Colloidal dispersions in which fine particles of a noble metal including silver have been dispersed in a solvent are conventionally used as a coating liquid for transparent electro-conductive layer formation used to prevent electromagnetic waves from leaking in computer displays (Japanese Patent Applications Laid-open No. H11-329071 and No. 2000-268639), a coating liquid for forming antimicrobial coatings (Japanese Patent Application Laid-open No. H4-321628), and so forth. For example, in the former use, a front glass panel of a cathode ray tube (CRT) is coated with the transparent electro-conductive layer forming coating fluid by spin coating or the like, followed by drying and thereafter baking at a temperature of about 200° C. to form a transparent electro-conductive layer.
A method is also proposed in which a highly concentrated fine silver particle colloidal dispersion (a paste) is applied by printing such as screen printing, followed by baking at a temperature of about 200° C. to obtain an electro-conductive silver layer (Japanese Patent Application Laid-open No. 2002-334618). However, the fine silver particle colloidal dispersion used for this purpose has ever been produced by an evaporation-in-gas method in which silver is evaporated and condensed in a gas under reduced pressure to collect the silver in a solution containing a dispersing agent. Hence, this method affords a very poor productivity, and therefore the fine silver particle colloidal dispersion thus obtained has been very expensive. Moreover, in the case of this fine silver particle colloidal dispersion, a dispersing agent capable of combining strongly with the surfaces of fine silver particles is contained in order to improve dispersion stability, and hence it is necessary to decompose and remove the dispersing agent by high-temperature heat treatment at about 200° C. after coating or printing and drying. This can not be said to be preferable.
Meanwhile, the Carey-Lea process (Am. J. Sci., 37, 38, 47, 1889) is available as a process by which a fine silver particle colloidal dispersion containing no dispersing agent can more simply be produced. In this Carey-Lea process, an aqueous silver nitrate solution is mixed in a mixed solution of an aqueous iron(II) sulfate solution and an aqueous sodium citrate solution to allow them to react, the resultant agglomerate of fine silver particles is filtered and washed, and thereafter pure water is added to the resultant cake, whereby a relatively highly concentrated fine silver particle colloidal dispersion can be obtained (Ag: 0.1 to 10% by weight). In regard to one process for obtaining coating liquid for silver film formations by using this silver particle colloidal dispersion, the present inventors have already made a proposal in PCT/JP2004/006053.
Fine silver particles obtained by the above Carey-Lea process are very fine nano-scale colloidal particles of 2 to 15 nm in particle diameter. In addition, in the course of actual production, where a commonly prevailing batch system is used, solutions tends to stand non-uniformly mixed when an aqueous solution of one raw material is put at a stretch into a container holding therein an aqueous solution of the other raw material, making it difficult to control particle diameters of the fine silver particles to be formed. Hence, coarse particles tend to come about in part. For example, coarse particles of about 30 nm in particle diameter may mix in silver particles of usually about 5 to 15 nm in particle diameter. This is remarkable especially when solutions are treated in a large quantity.
Accordingly, in order to make the aqueous raw material solutions mix and react uniformly with each other in the course of production to obtain a fine silver particle colloidal dispersion having a narrow particle size distribution, methods are recently proposed which are a method in which fine silver particles are continuously formed by means of a static mixer or the like while keeping constant the state of mixing and reaction of the aqueous raw material solutions (Japanese Patent Application Laid-open No. 2004-18891) and a method in which aqueous raw material solutions are respectively ejected from different nozzles to mix them (Japanese Patent Application Laid-open No. 2004-68072).
According to these methods, fine silver particle colloidal dispersions respectively having a narrow particle size distribution can be obtained, within the range of, e.g., 2 to 7 nm in an attempt to form particles with small particle diameter and within the range of, e.g., 10 to 15 nm in an attempt to form particles with large particle diameter. However, in these conventional methods making use of the Carey-Lea process, it has been unable to obtain a fine silver particle colloidal dispersion containing fine silver particles of more than 20 nm in average particle diameter however reaction conditions therefor are changed.
Thus, in the conventional methods making use of the Carey-Lea process, it has been unable to obtain any fine silver particle colloidal dispersion containing uniform fine silver particles of more than 20 nm in average particle diameter, and, accordingly, likewise in the coating liquid for silver film formation to be obtained from such a fine silver particle colloidal dispersion, no coating liquid has been obtained which contains uniform fine silver particles of more than 20 nm in average particle diameter.
Now, recently, where electro-conductive silver films are formed, it is desired to form them in a large layer thickness to make the films have a low electrical resistance. However, where coating liquids for silver film formation are used which contain conventional fine silver particles having a small average particle diameter, there has been a problem that cracks may occur at the time of film baking if films are made to have a large layer thickness of up to, e.g., several micrometers (μm), to cause a great deterioration of electro-conductivity and adhesion of the films.
To overcome this problem, it is necessary to keep the films from shrinking at the time of baking, stated specifically, it is necessary for the fine silver particles to have an average particle diameter of at least about 20 nm, or more. However, in the conventional Carey-Lea process, as stated above, it has been difficult to obtain the fine silver particle colloidal dispersion containing fine silver particles of more than 20 nm in average particle diameter. Accordingly, it has also been difficult to obtain at a low cost and in a simple way the coating liquid for silver film formation in which the fine silver particles of more than 20 nm in average particle diameter stand dispersed.